The object of the present invention is the preparation and use of highly porous conjugated polymer electrodes in electrochemical systems, as an electrode in a secondary battery, electrodes in a supercapacitor or electrocatalytic converter, or in sensor materials. The preparation route leads to more than 90% porous materials which show both high electronic and ionic conductivity, and are therefore useful for large effective contact area in supercapacitors; it also gives the function of faradaic charge transfer for charge storage in secondary batteries. The porous structure is also useful to enable charge transfer to soluble or immobilized redox species and is therefore useful as a contact in redox batteries, electrocatalytic systems and sensors.
Since electrochemistry essentially involves processes at the electrode-electrolyte interface, there has been a constant motivation for increasing the effective surface area of the electrodes. This is achieved by increasing the roughness or porosity of the electrodes. High surface area carbon electrodes have been fabricated from carbon pastes, fibrous carbon and carbon foams (S Sarangapani, B. V. Tilak, C.-P. Chen, J. Electrochem. Soc. 143 (1996) 379, J. D. Brown, R. F. Simandi U.S. Pat. No. 5,268,395 (1993)). Such materials have performed well as double layer capacitors and as support for the redox centers in electrodes for electrochemical capacitors, sensors and electrocatalysts (G. A. Hards, T. R. Ralph, U.S. Pat. No. 5,501,915 (1996); J. L. Kaschmitter, S. T. Mayer, R. L. Morrison, R. W. Pekala, WO 9423462 (1994)). The extreme case of this approach of increasing the surface area would be preparation of an electrode in which each molecule is in direct contact with the electrolyte, keeping electrical contact with the external circuit. Electrically conducting materials in gel state would have such a property. Fabrication of such materials has become a possibility with the advent of novel materials such as conducting polymers and carbon nano-tubes (C. M. Niu, E. K. Sichel, R. Hoch, D. Moy, H. Tennent, Appid. Phys. Lett., 70 (1997) 1480). In last two decades extensive studies have been done on electrochemical properties of conducting polymers (M. E. G. Lyons (ed.), Electractive Polymer Electrochemistry, pt 1, Plenum Press, New York (1994)). This kind of polymers often have good permeability to small molecules, leading to direct interaction of the bulk material with the electrolyte solutions (Y. M. Volfkovich, V. S. Bagotzky, E. Y. Pisarevskya, Electrochim. Acta, 41 (1996) 1905). They act as electroactive, at the same time, electron conducting material, when coated on an electrode. However, the mobility of ions in such materials is generally low, resulting in decrease in the reversibility of the electrode at high current densities (M. E. G. Lyons (ed.), Electroactive Polymer Electrochemistry, pt 1, Plenum Press, New York (1994)). Soluble conducting polymers have been stabilized in gel state (X. Chen, O. inganats, Synth. Met., 74 (1995) 159; S. Shakuda, T. Kawai, S. Morita, K. Yoshino, Japn. J. Appld. Phys. pt 1, 33 (1994) 4121). However, detail investigation on the effect of the gel nature of the material, on their electrochemical properties, is yet to be reported. Moreover, these polymers are not water soluble, hence, their gels can only be prepared in non-aqueous medium. Due to low conductivity of non-aqueous systems, such material are not suitable for preparation of high power electrodes. Conducting polymers have been grown using conventional hydrogeis to impart a gel like characteristics (K. Gilmore, A. J. Hodgson, B. Luan, C. J. Small, G. G. Wallace, Polym. Gels and Network, 2 (1994) 135; S. Ghosh, V. Kalpagam Synth. Met. 60 (1993) 133). However, in such systems the conducting polymer tend to agglomerate rather than forming a network structure, leading to poor electrical connectivity through out the material (X. M. Ren, P. G. Pickup, J. Electroanal. Chem., 396 (1995) 359). Composite of conducting polymers with graphite fibers have been prepared, which shows moderately good charge storage capacity as capacitor electrodes (B. Coffey, P. V. Madse, T. O. Poehier and P. C. Searson, J. Electrochem. Soc. 142 (1995) 321).
Apart from increasing the effective surface area, conducting gels, can impart various chemical characteristics to the electrodes more effectively than that can be done by surface modifications of solid electrodes. Such materials, with functionalized xe2x80x9cmolecular wiresxe2x80x9d stabilized as gel in the electrolyte solution, may lead to development of highly sensitive and fast sensors (M. E. G. Lyons, C. H. Lyons, C. Fitzgerald T. Bannon, Analyst 118 (1993) 361), electrocatalysts (G. A. Hards, T. R. Ralph, U.S. Pat. No. 5,501,915 (1996)), selective electromembranes etc. Further, conducting polymers have multiple oxidation states which leads to higher pseudocapacitance (B. E. Conway, J. Electrochem. Soc., 138 (1991) 1539) and hence, higher energy densities for capacitor electrodes.
One of the most direct application of porous electrodes have been in capacitor devices. Electrodes containing redox species may have very high capacitance valuesxe2x80x94even up to several hundreds of Farads per gram of the electrode material. Such capacitors are known as supercapacitors, which are used as the energy storing element in electronic equipment as memory backup for computers, camcorders, cellular phones etc. Supercapacitors may find application in the field of power electronics e.g. in UPS (uninterrupted power supply) and motor starters. The major driving force for the development of supercapacitors in USA have been their potential for application in electric vehicles (A. F. Burke, 36th Power Sources Conference, (June 1994) Cherry Hill, N.J.).
Porous activated carbon electrodes with or without embedded redox compound have been applied as the electrode material in supercapacitors having capacitance of several hundreds of Farad per gram of material (H. Shi, Electrochim. Acta, 41 (1996) 1633). Mixed valent metal oxides as electrode material provide maximum capacitance (B. E. Conway, J. Electrochem. Soc., 138 (1991) 1539; F. P. Malaspina, JP 6503924 (1994), U.S. Pat. No. 5,079,674 (1992)), but have the disadvantage of being costly. Though so far only the carbon and the metal oxide based electrodes have found commercial application, there is increasing interest in conducting polymers as new materials for supercapacitor electrodes (J. A. C. Carlberg, O. W. Inganas, SE 9602955 (1998); C. Li, K. K. Lian, H. Wu, U.S. Pat. No. 5,591,318 (1997); A. J. Rudge, J. P. Ferraris, S. Gottesfeld, U.S. Pat. Nos. 5,527,640 (1996) and 5,527,640 (1996)). This is based on the fact that in conducting polymers, the charge is stored in the whole bulk of the material (A. Rudge, J. Davey, I. Raistrick, S. Gottesfeld, J. Power Sources, 47 (1994) 89), rather than only on the surface as in carbon based electrodes. These polymers are also of low cost and easy to produce. Another merit of conducting polymers, which is yet to be fully capitalized on and the present work is a step in that direction, is the possibility that these polymers can be nano-structured into suitable morphology (C. R. Martin, Science, 266 (1994) 1961) for optimum energy and power density. As described above, conducting gels may lead to electrodes with maximum surface area. However, it should be noted that though the surface area of an electrode increases with a decrease in pore diameter in an electrode material, the total extractable charge at short time shows a more complex behavior (S. Sarangapani, B. V. Tilak, C. P. Chen, J. Electrochem. Soc. 143 (1996) 3791). It shows rise with the decrease in the diameter until an optimal value (in the range of a few nanometers) is reached and then shows a decreasing trend. Therefore, an electrode with pore diameter of intermediate size or containing a distribution of diameter sizes, are more desirable for fast capacitor electrodes.
The present work is centered on the idea of nano-structuring a conducting gel with optimum network morphology. This has been achieved by crosslinking conducting polymer microgel particles dispersed in an aqueous medium. PEDOT-PSS complex, with excess fixed negative charge and dispersed in aqueous media, was found to be the suitable compound for this preparation. The dispersion is commercially available by the trade name, Batron-P, from Bayer AG, Germany. Because of the network structure and due to the gel nature of the particles used as its building block, the gel material prepared provides excellent access of the electrolyte solution to the polymer chains. The electrical connectivity of the polymer chains is further improved by electropolymerization of another conducting polymer, such as polypyrrole (PPy), polyaniline (PAni), using the polymer gel as the template. Since the gel network acts as a three dimensional electrode, the electropolymerized polymer acquires a three dimensional morphology. Presence of more than one conducting polymer in a material has the advantage of providing electrical conductivity to the material even when one of the polymers is in the non-conducting, undoped state. The latter polymer also improves mechanical stability of the electrode material. The idea presented here, provides flexibility in choosing the second or subsequent materials to be deposited on the initial physically crosslinked gel to impart desired properties function to the electrode. For example, a functional polymer may be used as a component to impart specific chemical characteristics to the material to be applied to sensors or electrocatalytic electrodes. Other water soluble components can be distributed in the composite material even at the time of the physical crosslinking of the dispersion. The PEDOT-PSS, blended with water soluble polymer, polyethyleneoxide (PEO), and physically crosslinked, has been found to form a network structure. This can be applied to solid state electrodes with high effective surface area.